Tyros 5 for sure.
Why?
A simple answer could be: The aftertouch and the better keybed of the Tyros series.
Those two have a big impact on how a sample sounds between two different models in my opinion.
I don't know what else is inside a keyboard, from a technical point of view, that affects the quality of the sound.
Maybe there are very few new samples is PSX900 that Tyros 5 don't have,
but the missing voices of the SX900 compared to Tyros are more ( scat voices,orchestrated sounds etc.
It's kind like asking if a the newest A class Mercedes can be better than the previous C class.
The new SX is a great upgade from an older psr S or for new buyers but not for those who have a Tyros 5 and want a better sound.
A little better sound,better and newer styles and a lot of stuff that are better from Tyros 5, has only the Genos.